High sensitivity magnetometers, including zero-field paramagnetic resonance magnetometers (ZF-PRM) as described by Slocum and Reilly in 1963 (Slocum and Reilly 1963), have a wide range of applications including, but not limited to fundamental research, detecting biomagnetic signals such as those emanating from biological organisms including the human body, geophysical exploration and prospecting, navigation and space applications, and military uses such as ordinance and underground-underwater structure detection. Until recently, the most sensitive commercially available magnetometer for such applications was the superconducting quantum interference device (SQUID) (Weinstock 1996). However, zero-field paramagnetic resonance magnetometers (Dupont-Roc, Haroche, and Cohen-Tannoudji 1969; Marie-Anne et al. 1971; Vishal Shah et al. 2007), which have advanced to comparable sensitivity as SQUID systems, have recently gained popularity as a lower-cost, more robust alternative to SQUID magnetometers for many applications.
Significant developments in alkali atomic magnetometery (Budker and Romalis 2007) over the past decade have led to a variety of techniques and methods for sensing magnetic fields. In general, the different methods are based on the same fundamental physical sensing mechanism that exploits the energy structure of atoms and the perturbations that result in their energy levels, or spin state, from exposure to external magnetic fields. Atomic based magnetic sensors measure the direction and magnitude of an external magnetic field through the induced changes in the atomic spin polarization of an ensemble of atoms.
A zero field parametric resonance magnetometer (ZF-PRM) relies on detecting changes in optical transmission properties of spin polarized atomic vapor in a narrow magnetic field range centered on absolute zero magnetic field. The optical transmission properties of the polarized atomic vapor in ZF-PRM change substantially when magnetic field, in a direction perpendicular to the direction of the atomic spin polarization, changes. However, little or no change is observed when there is a change in magnetic field parallel to the direction of the atomic spins. Consequently, ZF-PRM is primarily sensitive to magnetic field in just one or two orthogonal directions perpendicular to the atomic spin polarization, and not sensitive to magnetic field in the third direction which is parallel to the direction of the atomic spin polarization. In many scientific, research, and biomedical applications, measurement of all three orthogonal vector components of the magnetic field is necessary.